SIDE IvIGHTS ON BIRDS 



as well as the construction of the feathers, all 

 serve to render the bird-form especially suitable 

 to journey through the air. 



Another peculiarity admirably adapted to assist 

 flight is the formation of the bird's neck. This 

 differs from that of mammals in both length and 

 flexibility, for whereas the terrestrial forms have 

 only seven cervical vertebrae, most birds have 

 fourteen, and the swan twenty -three. The ad- 

 vantage of this is obvious. It enables the bird 

 in flying to bring the head so far forward that the 

 centre of gravity is fixed at the point directly 

 below where the wings join the body, thus giving 

 a perfect balance. Further, the lower muscles 

 of the bird's neck bend most easily backwards, 

 while the upper muscles bend naturally forward. 

 Thus, when at rest the neck takes the shape of 

 the letter S as seen most plainly in the graceful 

 form of the swan. This is the position assumed 

 for walking, as it alters the centre of gravity back 

 again to a point above the base of the toes. 

 One striking example of the flexibility of the neck 

 is seen in the wryneck, which is able to turn its 

 head completely round. A further remarkable 

 provision of nature is in the formation of the legs 

 and feet of birds. A stork is able to stand on 

 one leg for whole days without fatigue, the reason 

 being that the legs are kept out-stretched without 

 any exertion of the muscles. If the legs of a 

 dead fowl are bent downwards it will be observed 

 that the claws close of their own accord and re- 

 main locked. This enables birds to go to sleep 

 at night with claws clasped automatically on the 

 branches, and the hold is retained until the 

 percher, by a voluntary effort, releases it. 



The feather is composed of three parts : the 

 quill, the shaft, and the vane ; the quill firmly 



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